Integrated tip strain sensor for use in combination with a single axis atomic force microscope

ABSTRACT

An integrated tip strain sensor is combination with a single axis atomic force microscope (AFM) for determining the profile of a surface in three dimensions. A cantilever beam carries an integrated tip stem on which is deposited a piezoelectric film strain sensor. A high-resolution direct electron beam (e-beam) deposition process is used to grow a sharp tip onto the silicon (Si) cantilever structure. The direct e-beam deposition process permits the controllable fabrication of high-aspect ratio, nanometer-scale tip structures. A piezoelectric jacket with four superimposed elements is deposited on the tip stem. The piezoelectric sensors function in a plane perpendicular to that of a probe in the AFM; that is, any tip contact with the linewidth surface will cause tip deflection with a corresponding proportional electrical signal output. This tip strain sensor, coupled to a standard single axis AFM tip, allows for three-dimensional metrology with a much simpler approach while avoiding catastrophic tip &#34;crashes&#34;. Two-dimensional edge detection of the sidewalls is used to calculate the absolute value or the linewidth of overlay, independent of the AFM principle. The technique works on any linewidth surface material, whether conductive, non-conductive or semiconductive.

CROSS-REFERENCE TO RELATED APPLICATIONS

This patent application is related to U.S. patent application Ser. No.07/568,451, filed Aug. 16, 1990, entitled "Method of Producing UltrafineSilicon Tips for the AFM/STM Profilometry", U.S. patent application Ser.No. 07/608,043, filed Oct. 31, 1990, entitled "Nanometer Scale Probe foran Atomic Force Microprobe, and Method for Making Same", U.S. patentapplication Ser. No. 07/619,378, entitled "Microprobe-Based CDMeasurement Tool", and U.S. patent application Ser. No. 07/767,300,filed Sep. 27, 1991, entitled "Combined Scanning Force Microscope andOptical Metrology Tool", and which are all assigned to the same assigneeas this application, and the disclosure of all of them is incorporatedherein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention generally relates to metrology of linewidths,trenches and overlays on a submicron range using strain sensors and,more particularly, to probe assembly used in combination with an AtomicForce Microscope (AFM) for making measurements in three dimensions. TheAFM is used to make measurements in the z direction and then the probeassembly is used in the x-y plane perpendicular to the z direction toprofile a sample.

2. Description of the Prior Art

Quality control in the manufacture of integrated circuits (ICs) requiresa technology for measuring surface geometries of the IC wafer. One suchtechnology employs a small thin piezoelectric member affixed to a largermounting member to which there is attached a probe tip. The probe tip ismade to contact the surface being mapped thereby causing a flexing ofthe mounting member and a corresponding flexing of the piezoelectricmember, producing an electrical signal. An example of this technology isshown in U.S. Pat. No. 4,888,550 to Reid which discloses a multiprobetip assembly used for quality control testing of an integrated circuitwafer. The Reid device uses a strain sensing piezoelectric element atthe base supporting the entire tip structure and, because of thisconstruction, is limited to sensing only in the vertical direction.

In practice, the contact tip technology as known in the prior art hasbeen useful in measuring micron scale or larger structures. Furthermore,the sensitivities of these structures has been relatively low, makingthem unsuitable for measurements of submicron scale structures such asthose of the newer gigabit chips.

Scanning Tunneling Microscopy (STM) technology introduced a new conceptin measurements using a probe type approach to map the surfaces ofintegrated circuit structures. The basic STM is disclosed in U.S. Pat.No. 4,343,993. Briefly described, a sharply pointed, electricallyconductive tip is placed at a distance on the order of one nanometerfrom the conductive surface of a sample to be investigated, with anappropriate electrical potential applied across the gap between the tipand surface. As the electron clouds of the atoms at the apex of the tipand the surface touch, a flow of electrons will result giving rise to atunneling current which happens to be extremely sensitive to changes ingap width. To render these changes as close as possible to zero, afeedback control system controls the distance of the tip from thesurface, using deviations of the tunneling current from an initial valueas a control signal. This control signal is also employed to generate aplot of the topography of the surface being investigated.

Atomic Force Microscopy (AFM) is a variation of the STM technology. Inone design, a sensor consisting of a spring-like cantilever which isrigidly mounted at one end and carries at its free end a dielectric tip,profiles the surface of an object. The force between the object'ssurface and the tip deflects the cantilever, and this deflection can beaccurately measured. A spatial resolution of 3 nm has been achieved.

Another version of the AFM includes optical detection instead of STMdetection. In this version, a tungsten tip at the end of a wire ismounted on a piezoelectric transducer. The transducer vibrates the tipat the resonance frequency of the wire which acts as a cantilever, and alaser heterodyne interferometer accurately measures the amplitude of thea.c. vibration. The gradient of the force between the tip and the samplemodifies the compliance of the lever, hence inducing a change invibration amplitude due to the shift of the lever resonance. Knowing thelever characteristics, one can measure the vibration amplitude as afunction of the tip-to-sample spacing in order to deduce the gradient ofthe force and, thus, the force itself.

A most critical component of the AFM is the spring-like cantilever. As amaximum deflection for a given force is needed, a cantilever is requiredwhich is as soft as possible. At the same time, a stiff cantilever witha high natural frequency is necessary in order to minimize thesensitivity to vibrational noise from the building. For meeting bothrequirements, dimensions of the cantilever beam are necessary that canonly be obtained by microfabrication techniques. Another most criticalcomponent of the AFM is the tip itself. Tip "crashes" are the mostlikely failure cause of AFM tips, and such failures are catastrophic,requiring the replacement of the cantilever beam and tip subassembly.This is perhaps the greatest limitation to the use of AFM.

While STM and AFM both have the potential capability for profiling on anatomic diameter scale, the difficulty with both is the measurement oftrenches in two dimensions. Both require sophisticated multi-connectedinterferometers to detect the two-dimensional tip motion and, for AFM tobe useful as a reliable and accurate metrology tool, the sensor tipshould be highly reproducible in the fabrication process and the tipshape should be known with high accuracy. Moreover, for profiling andcritical dimension (CD) measurement of silicon (Si) memory trenches withsubmicron trenches with depths of 1 μm or more, nanometer-scale tipshapes with high aspect ratio will be required.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide ameasurement technology which is capable of meeting the advancedmetrology requirements imposed by the dimensions of gigabit chips.

It is another, more specific object of the invention to provide a strainsensor device in combination with an AFM that is capable ofthree-dimensional measurements of submicron geometries with highsensitivity.

It is a further object of the invention to provide an AFM measurementsystem which protects the tip of the cantilever from catastrophic"crashes".

According to the invention, there is provided a cantilever beam carryingan integrated tip stem on which is deposited a piezoelectric film strainsensor. A high-resolution direct electron beam (e-beam) depositionprocess is used to grow a sharp tip onto a silicon (Si) cantileverstructure. The direct e-beam deposition process permits the controllablefabrication of high-aspect ratio, nanometer-scale structures. Apiezoelectric jacket with four electrical contacts superimposed isdeposited on the tip stem. The piezoelectric sensors function in a planeperpendicular to that of the probe in STM; that is, any tip contact withthe linewidth surface will cause tip deflection with a correspondingproportional electrical signal output. This tip strain sensor, coupledto a standard single axis AFM tip, allows for three-dimensionalmetrology with a much simpler approach while avoiding catastrophic tip"crashes". The potential for a tip "crash" is detected by the electricalsignal output from the strain sensor exceeding a predeterminedthreshold. When the threshold is exceeded, the feedback control systemstops the movement of the tip and then backs the tip away from a surfacefeature with which it has come into contact. Two-dimensional edgedetection of the sidewalls is used to calculate the absolute value ofthe linewidth or overlay, independent of the AFM principle. Thetechnique works on any linewidth surface material, whether conductive,non-conductive or semiconductive.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other objects, aspects and advantages will be betterunderstood from the following detailed description of a preferredembodiment of the invention with reference to the drawings, in which:

FIG. 1 is an isometric view showing a scanning probe tip attachment fora one-dimensional AFM system combined with a two-dimensional integratedtip strain sensor element according to the invention;

FIG. 2 is a cross-sectional view of the tip element shown in FIG. 1showing the piezoelectric film divided into four quadrants and providedwith overlying electrical contacts;

FIG. 3 is a graph showing a characteristic voltage output of the strainsensor elements as a function of tip lateral displacement in the X-axisand/or the Y-axis;

FIG. 4 is a block diagram showing the AFM system combined with theintegrated tip strain sensor element; and

FIG. 5 is an enlarged view, partially in cross-section, showing theintegrated tip strain sensor element within a trench and illustratingthe operation of the system.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION

Referring now to the drawings, and more particularly to FIG. 1, there isshown the scanning probe tip attachment 10 for a one-dimensional AFMsystem (not shown). Starting with a standard cantilever beam 11 with anintegrated tip stem 12, fabricated by micromachining techniques, theextended tip stem 15 and tip structure 16 is deposited by establishede-beam techniques described, for example, by K. L. Lee and M. Hatzakisin "Direct electron-beam patterning for nanolithography", J. Vac. SciTechnology, November/December 1989, pp. 1941-1946. A piezoelectric film17 is deposited on the tip stem 12 and divided into four quadrants bye-beam etching to define four piezoelectric sensors 17₁, 17₂, 17₃, and17₄. The preferred implementation of this invention is to use leadzirconate titanate (PZT) material as the piezoelectric film 17 toconstruct the strain sensor elements 17₁, 17₂, 17₃, and 17₄, shown inmore detail in FIG. 2. PZT material can be deposited with a variety ofdifferent processes known in the art, including rf-magnetron sputteringand chemical methods. Metal film is patterned to provide electricalconductors 18₁, 18₂, 18₃, and 18₄ which make electrical contact withrespective strain sensor elements 17₁, 17₂, 17₃, and 17₄ and extend upthe tip stem 12 and along the under surface of the cantilever beam 11,and from there electrical connections 19₁, 19₂, 19₃, and 19₄ are made tothe measuring circuitry (not shown). The common base electrode for thestrain sensor elements 17₁, 17₂, 17₃, and 17₄ can be either the tip stem12 itself or a conductive coating, preferably Pt, or a conductive oxide.In either case, an insulative coating is provided to separate theconductive stem or coating from the electrical conductors 18₁, 18₂, 18₃,and 18₄. Additionally, a conductive path is provided as the common orreturn path to the measuring circuitry. The conductive coating, if used,and the corresponding common or return path are not shown in thedrawings in order to simplify the illustration, but those skilled in theart will understand that the connection is implied.

The implementation of the strain sensor elements 17₁, 17₂, 17₃, and 17₄can be accomplished using different geometrical configurations, theconfiguration shown in FIGS. 1 and 2 being preferred. One specificreduction to practice is described below:

1. The stem 12 is first prepared by depositing an insulative layer ofoxide, if the stem itself is the conductive return path for the sensors,or creating a conductive return path by depositing layers of aconductive coating (e.g., Pt or a conductive oxide) and an insulativelayer.

2. A continuous PZT coating 17 is applied to the tip stem 15 and etchedto form the four quadrant structure shown.

3. The deposited material is heat treated in an oxygen containingenvironment to crystalline into the proper PZT phase and to oxidize theexposed cantilever structure 11 and tip stem 12, achieving an electricalinsulating coating on the same.

4. A continuous metal film is applied onto PZT coating 17 and at leastthe bottom surface of the cantilever beam 11 as an overcoat.

5. The metal film is patterned to yield the four quadrant contacts onthe PZT coating 17 and the electrical conductors 18₁, 18₂, 18₃, and 18₄which extend up the tip stem 12 and along the bottom length of thecantilever beam 11 to provide the output signal on lines 19₁, 19₂, 19₃,and 19₄ from the strain sensor contacts.

The patterning of the metal film into quadrant contacts and conductinglines is most conveniently accomplished by laser ablation, ion millingor e-beam processing.

As shown in FIG. 1, the tip 16 extends into a trench 21 formed in thesurface of a chip 20 which is to be profiled. The trench 21 has sidewalls 22 and 23 and a bottom 24. In operation, the system starts up asan AFM probe vibrating the tip 16 in the z direction and moving the tipfrom the surface to the bottom 24 of the trench 21. Vibration of thecantilever beam 11 is produced by a bimorphous crystal 26, and trenchdepth is defined by the AFM viewing the vibration of the end of thecantilever beam 11 via lens 27. At this point, tip vibration isterminated, the z dimension of the trench having been established. Thetip is backed off from the surface (e.g., 20 to 80% of depth). The sidewalls are located by moving the tip in the x-y directions until strainis detected by the piezoelectric sensors due to tip deflection. FIG. 3shows the voltage output for one of the piezoelectric sensors as afunction of deflection of the tip 16. By a simple geometric calculationbased on the relative amplitudes of the signals from each of the foursensors 17₁, 17₂, 17₃, and 17₄, the x and y coordinates are defined, andthe tip 16 is moved to the opposite sidewall. When detected, the x and ycoordinates of the opposite sidewall are also defined. Then, again usinga simple geometric calculation, the linewidth or overlay is calculated.

FIG. 4 shows in block diagram form the overall system including the AFM.The chip 20 is mounted on a stage 30 for movement in the x-y plane. TheAFM itself includes, in addition to the lens 27, a laser heterodynesystem 31, the output of which is the z feedback signal to an amplifier32 which provides an a.c. driving signal from generator 33 to thebimorphous crystal 26. The z feedback signal is also supplied to amicroprocessor controller 34 which controls the application of thedriving signal to the crystal 26, this being terminated when the zdimension has been determined. After the z dimension has beendetermined, the tip 16 is withdrawn to a predetermined distance abovethe bottom of the trench and moved relative to the trench in the x-yplane. This is accomplished by the controller 34 moving the stage 30. Afeedback loop defined by the output signals from the sensors 17₁, 17₂,17₃, and 17₄ to the controller 34 is used to control relative tipmotion. The three dimensions determined in this fashion are output bythe controller 34 to a suitable output device 35 which displays, printsand/or stores the data.

A most valuable and important part of the invention is its ability toprevent "crashes" of the tip. "Crash" prevention is accomplished by themicroprocessor controller 34 including a programmed routine whichcompares the absolute value of the voltage output from the sensors 17₁,17₂, 17₃, and 17₄ with a predetermined threshold. As generally indicatedin FIG. 3, the absolute value of voltage is approximately a linearfunction of tip deflection, and the threshold is selected to correspondto a tip deflection well within elastic limit of the tip 15. The routinesimply compares each of the outputs of the sensors 17₁, 17₂, 17₃, and17₄ with the predetermined threshold. An alternative, although morecomplex routine, performs a simple trigonometric calculation on the twopositive and two negative voltages from the sensors to resolve thedirection of the tip deflection. Such a routine is part of the profilingcomputation performed by controller. The resolved absolute value ofvoltage amplitude is then compared to the threshold. When the thresholdis exceeded, the controller 34 stops the motion of the tip 15 relativeto the sample being profiled and then backs the tip away from thesurface feature with which the tip has come into contact.

The preferred measurement regimen is illustrated in FIG. 5. After the zdimension is determined, the tip 16 is first withdrawn by 20% of thetrench depth, and the x and y coordinates are determined for point A onside wall 22 and then point D on side wall 23. Then the tip 16 iswithdrawn by 80% of the trench depth, and the x and y coordinates aredetermined for point B on side wall 22 and then point C on side wall 23.The calculated width of the trench 21 is based on the root mean squareof the width AD and the width BC.

While the invention has been described in terms of a single preferredembodiment, those skilled in the art will recognize that the inventioncan be practiced with modification within the spirit and scope of theappended claims.

Having thus described our invention, what we claim as new and desire tosecure by Letters Patent is as follows:
 1. An integrated tip strainsensor in combination with an atomic force microscope (AFM) forprofiling a surface in three dimensions comprising:a cantilever beamcarrying a tip stem having a tip, said tip stem being an integrated tipstem; a piezoelectric film strain sensor on said integrated tip stem forsensing strain; and control means for selectively vibrating saidcantilever in a first direction, said piezoelectric film strain sensorproducing electrical signals in response to deflections of said tip dueto contact of said tip with a surface feature, wherein saidpiezoelectric film strain sensor comprises a plurality of portions aboutsaid tip stem and means electrically connected to each of said portionsfor providing a feedback control signal to said control means.
 2. Theintegrated tip strain sensor as recited in claim 1 wherein saidpiezoelectric film strain sensor comprises four quadrant portions aboutsaid tip stem.
 3. The integrated tip strain sensor as recited in claim 2wherein said cantilever beam and tip stem are made of silicon and ahigh-resolution direct electron beam (e-beam) deposition process is usedto grow said tip onto said tip stem.
 4. An integrated tip strain sensoras recited in claim 1, wherein said atomic force microscope comprises anatomic force microscope having at least one axis.
 5. An integrated tipstrain sensor as recited in claim 1, wherein said control means includesmeans for vibrating said cantilever in a z direction, said AFM includingmeans for detecting cantilever vibrations to measure a distance in saidz direction to a first surface feature, said control means thereafterstopping cantilever vibrations and relatively moving said tip withrespect to said surface, said piezoelectric film strain sensor includingmeans for producing electrical signals in response to deflections ofsaid tip due to a second surface feature.
 6. The integrated tip strainsensor recited in claim 5 wherein said control means includes thresholdmeans for comparing said electrical signals generated by saidpiezoelectric film strain sensor in response to deflections of said tip,said threshold means stopping relative movement of said tip with respectto said surface to avoid a "crash" of said tip.
 7. An integrated tipstrain sensor as recited in claim 5, wherein said means for producingelectrical signals includes metallized contacts electrically connectedto each of said plurality of portions of said piezoelectric film strainsensor.
 8. An integrated tip strain sensor as recited in claim 7,wherein said metallized contacts include means for providing a feedbackcontrol signal to said control means.
 9. A probe for a metrology toolfor measuring critical dimensions in a sample comprising:a tip stem onwhich is grown a tip, said tip stem being an integrated tip stem,wherein said integrated tip stem includes a piezoelectric film strainsensor, said piezoelectric film strain sensor having a plurality ofportions and including means for generating electrical signals inresponse to respective deflections of said plurality of portions;control means for moving said tip with respect to said surface; andfeedback means including said control means responsive to saidelectrical signals generated in response to said respective deflectionsof said plurality of portions of said piezoelectric film strain sensorfor controlling the movement of said tip.
 10. The probe recited in claim9 wherein said control means includes threshold means for comparing saidelectrical signals generated by said piezoelectric film strain sensor inresponse to deflections of said tip, said threshold means stoppingrelative movement of said tip with respect to said surface to avoid a"crash" of said tip.
 11. The probe recited in claim 9 further comprisingstrain sensor means for sensing deflection of said tip, said feedbackmeans including said strain sensor means.
 12. The probe recited in claim11, wherein said strain sensor means includes said piezoelectric filmstrain sensor, said piezoelectric film strain sensor being deposited onsaid integrated tip stem.
 13. An integrated tip strain sensor incombination with an atomic force microscope (AFM) for profiling asurface in three dimensions comprising:a cantilever beam carrying a tipstem having a tip, said tip stem being an integrated tip stem; and apiezoelectric film strain sensor on said integrated tip stem for sensingstrain; control means for vibrating said cantilever in a z direction,said AFM detecting cantilever vibrations to measure a distance in said zdirection to a first surface feature, said control means thereafterstopping cantilever vibrations and relatively moving said tip withrespect to said surface, said piezoelectric film strain sensor producingelectrical signals in response to deflections of said tip due to saidsecond surface feature, said control means x and y coordinates for saidsecond feature, wherein said piezoelectric film strain sensor comprisesfour quadrant portions about said tip stem.
 14. The integrated tipstrain sensor as recited in claim 13, further comprising metallizedcontacts electrically connected to each of said quadrants and providinga feedback control signal to said control means, said cantilever beamand tip stem are made of silicon and a high-resolution direct electronbeam (e-beam) deposition process is used to grow said sharp tip ontosaid tip stem.